1. Field of the Invention
This invention relates to arrangements and devices for protecting electrical circuits from overcurrents, for example from overcurrents caused by equipment faults, electrostatic discharge or other threats.
2. Introduction to the Invention
One circuit protection arrangement of relatively simple form is described in German Patent Application No. 37 25 390 dated Jul. 31, 1987 to Wickmann-Werke GmbH. This arrangement comprises a series switching transistor that controls the circuit current and a control transistor that controls the base or gate voltage of the switching transistor. The base or gate voltage of the control transistor is set by a voltage divider that spans the switching transistor, so that, if the arrangement experiences an overcurrent, the control transistor will be biased into conduction and will turn the switching transistor OFF. Although this device is particularly simple, it suffers from the disadvantage that its performance can vary considerably with temperature. For example, in many cases the current required to open the switching transistor (the "trip current") will fall to a small fraction of the ambient trip current when the temperature rises to 100.degree. C. or so. Thus, in normal operation of the circuit, it is usually only possible to pass a very small fraction, eg. about 20%, of the trip current through the switch. If larger currents are allowed to pass, heating of the switching transistor (due to a voltage drop across it that is always in the order of a few volts) will cause the trip current level to fall, with the result that tripping of the arrangement will at some stage occur with the normal circuit current.